Mechanisms of hydrogenation. Part X. Homogeneous hydrogenolysis of organic halides and hydrogenation of the carbonyl, epoxy-, and nitro-groups

Abstract

The catalyst formed from trichlorotris(pyridine)rhodium with sodium borohydride in dimethylformamide is shown to be effective for the homogeneous hydrogenolysis of the carbon–halogen bond, R–X, where R = benzyl, allyl, aryl, or –CO–C but not alkyl, and X = Cl or Br. Vicinal dihalides have also been hydrogenolysed. The stereochemistry of halogen displacement is consistent with a free-radical mechanism. The carbonyl groups in acetophenone, benzophenone, and benzoin have been hydrogenated with this catalyst system, and styrene oxide gives 2-phenylethanol. Examples of hydrogenation of nitro-groups (RNO₂→ RNH₂; R = aryl or cyclohexyl) are reported.